AFCI Basics

Arc-Fault Circuit Interrupter (AFCI) protection has been a requirement since the 1999 National Electrical Code (NEC), starting as protection for receptacles in bedrooms and then progressing to protection of all outlets in bedrooms with changes in later Code cycles. In the 1999 NEC and the 2002 NEC, the method of AFCI protection was the branch/feeder AFCI, which transitioned to a combination AFCI, as found in 210.12(B) of the 2005 as a Jan. 1, 2008, requirement. The 2008 NEC holds further sweeping changes in AFCI protection, and a thorough understanding of the ins and outs of AFCI protection will help the electrical contractor and electrician in dealing with these changes.

A brief explanation of the major differences between the branch/feeder AFCI device and the combination device will detail the reasoning behind the transition and help explain the rationale for the switch to the combination devices.

An AFCI is a device intended to provide protection from the effects of arc faults by recognizing characteristics unique to arcing and by functioning to de-energize the circuit when an arc fault is detected. In addition to recognizing arcing characteristics, a branch/feeder AFCI circuit breaker has short-circuit protection (a short between two ungrounded conductors or between an ungrounded conductor and a grounded or neutral conductor). These devices also have overload protection for the circuit. Prolonged overloading of the circuit causes heating, which subsequently damages conductor insulation and can cause possible damage to equipment and other components in the electrical circuit.

A branch/feeder AFCI is designed to handle parallel faults in a circuit, such as a fault in an NM cable where a staple driven too deep into the wood stud damaged the hot and neutral conductors within the cable, causing arcing with a level of approximately 75 amperes or higher. (The new combination AFCI devices will require an arc level of approximately 5 amperes.)

In addition to the ability to clear a parallel fault, branch/feeder AFCI circuit breakers also have a ground-fault protection function in the circuit breaker that provides ground-fault protection starting at approximately 30 milliamps. This ground-fault protection is not personal protection, as would be the case for a GFCI function with tripping levels of between 4 and 6 milliamps or higher. Rather, it is equipment protection and often has been the cause for misdiagnosing the cause of an AFCI trip. For example, a branch circuit supplying electronic equipment with high amounts of leakage current to the equipment-grounding conductor could cause tripping of the ground-fault protection.

In addition, where electronic circuitry has been designed to use the equipment-grounding conductor as the functional current path for the effective operation of the equipment, the ground-fault protective device within the AFCI circuit breaker will trip but not due to any arcing faults.

Recently, an electrical contractor informed me that he was installing electronic dimmer modules, and when all recommended modules were connected to the protected circuit, the AFCI breaker would trip. The manufacturer of the dimmer modules suggested that the electrical contractor reduce the number of modules connected to the circuit to a maximum 1,000-VA load on a 20--ampere branch circuit, and the AFCI device would not trip. He limited the dimmer module load to 1,000 VA, and the AFCI device did not trip, solving the problem.

As mentioned, a branch/feeder AFCI is designed primarily for parallel faults, whereas a combination AFCI device is designed to function for both parallel and series faults. An example of a series fault would be an extension cord with a damaged stranded conductor within the cord. A series AFCI device detects the arcing current of 5 amps or more through the copper strands, thereby protecting the circuit and averting a possible fire. Tripping at 5 amps or more results in series AFCI devices being 15 times more sensitive to both parallel and series-arcing faults.

Bathroom, kitchen and garage lighting, as well as any other lighting in areas not protected by AFCI devices, can be connected to those circuits protected by AFCI devices, even though those outlets are not required to be AFCI protected. Spending some time planning the layout for the dwelling unit branch circuits in these areas will help maximize the protection for outlets while not substantially increasing the cost for the dwelling unit.

ODE is a staff engineering associate at Underwriters Laboratories Inc., in Research Triangle Park, N.C. He can be reached at 919.549.1726 or at mark.c.ode@us.ul.com.

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